Nanowire LED structure and method for manufacturing the same

ABSTRACT

A light emitting diode (LED) structure includes a plurality of devices arranged side by side on a support layer. Each device includes a first conductivity type semiconductor nanowire core and an enclosing second conductivity type semiconductor shell for forming a pn or pin junction that in operation provides an active region for light generation. A first electrode layer extends over the plurality of devices and is in electrical contact with at least a top portion of the devices to connect to the shell. The first electrode layer is at least partly air-bridged between the devices.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to nanowire light emitting diodesstructures, in particular arrays of nanowire light emitting devices andin particular to contacting thereof.

BACKGROUND OF THE INVENTION

Light emitting diodes (LEDs) are increasingly used for lighting, butstill there are some technological challenges to overcome, in particularwith regard to large-scale processing, in order to reach the realbreakthrough.

Over recent years the interest in nanowire technology has increased. Incomparison with LEDs produced with conventional planar technologynanowire LEDs offer unique properties due to the one-dimensional natureof the nanowires, improved flexibility in materials combinations due toless lattice matching restrictions and opportunities for processing onlarger substrates. Suitable methods for growing semiconductor nanowiresare known in the art and one basic process is nanowire formation onsemiconductor substrates by particle-assisted growth or the so-calledVLS (vapor-liquid-solid) mechanism, which is disclosed in e.g. U.S. Pat.No. 7,335,908. Particle-assisted growth can be achieved by use ofchemical beam epitaxy (CBE), metalorganic chemical vapour deposition(MOCVD), metalorganic vapour phase epitaxy (MOVPE), molecular beamepitaxy (MBE), laser ablation and thermal evaporation methods. However,nanowire growth is not limited to VLS processes, for example the WO2007/102781 shows that semiconductor nanowires may be grown onsemiconductor substrates without the use of a particle as a catalyst.One important breakthrough in this field was that methods for growinggroup III-V semiconductor nanowires, and others, on Si-substrates havebeen demonstrated, which is important since it provides a compatibilitywith existing Si processing and non-affordable III-V substrates can bereplaced by cheaper Si substrates.

One example of a bottom emitting nanowire LED is shown in WO 2010/14032.This nanowire LED comprises an array of semiconductor nanowires grown ona buffer layer of a substrate, such as a GaN buffer layer on a Sisubstrate. Each nanowire comprises an n-type nanowire core enclosed in ap-type shell and a p-electrode with an active layer formed between then-type and p-type regions that form a pn or pin junction. The bufferlayer has the function of being a template for nanowire growth as wellas serving as a current transport layer connecting to the n-typenanowire cores. Further the buffer layer is transparent since the lightthat is generated in the active area is emitted through the bufferlayer.

Although having advantageous properties and performance the processingwith regard to contacting of the nanowire LEDs requires new routes ascompared to planar technology. Since nanowire LEDs comprise large arraysof nanowires, thereby forming a three-dimensional surface with highaspect ratio structures, deposition of contact material usingline-of-sight processes is a challenging operation.

SUMMARY OF THE INVENTION

In view of the foregoing one object of embodiments of the invention isto provide improved nanowire LEDs and new routes for contacting thereof.

This object is achieved by a semiconductor device and a method forforming a semiconductor device in accordance with the independentclaims.

A nanowire light emitting diode (LED) structure in accordance withembodiments of the invention comprises nanowires arranged side by side.Each nanowire comprises a first conductivity type (e.g., n-type)nanowire core and an enclosing second conductivity type (e.g., p-type)shell for forming a pn or pin junction that in operation provides anactive region for light generation. While the first conductivity type ofthe core is described herein as an n-type semiconductor core and thesecond conductivity type shell is described herein as a p-typesemiconductor shell, it should be understood that their conductivitytypes may be reversed. A p-electrode layer extends over a plurality ofnanowires and is in electrical contact with at least a top portion ofthe nanoelements to connect to the p-type shell. The p-electrode layeris at least partly air-bridged between the nanowires.

Traditional, planar LEDs comprise functional layers in a sandwichstructure. In their simplest form, the planar LEDs comprise at leastthree functional layers: a p-doped layer, an active region, and ann-doped layer. Functional layers may also include wells, barriers,intrinsic and graded layers (e.g., as part of the active region). TheLED arrays described in embodiments of the invention distinguishthemselves by at least one of the functional layers being electricallyseparated from the surrounding LEDs in the array. Another distinguishingfeature is the utilization of more than one facet and non-planarity offunctional layers as emission layers.

Although the fabrication method described herein preferably utilizes ananowire core to grow semiconductor shell layers on the cores to form acore-shell nanowire, as described for example in U.S. Pat. No.7,829,443, to Seifert et al., incorporated herein by reference for theteaching of nanowire fabrication methods, it should be noted that theinvention is not so limited. For example, as will be described below, inthe alternative embodiments, only the core may constitute thenanostructure (e.g., nanowire) while the shell may optionally havedimensions which are larger than typical nanowire shells. Furthermore,the device can be shaped to include many facets, and the area ratiobetween different types of facets may be controlled. This is exemplifiedin figures by the “pyramid” facets and the vertical sidewall facets. TheLEDs can be fabricated so that the emission layer formed on templateswith dominant pyramid facets or sidewall facets. The same is true forthe contact layer, independent of the shape of the emission layer.

The use of sequential (e.g., shell) layers gives that the finalindividual device (e.g., a pn or pin device) may have a shape anywherebetween a pyramid shape (i.e., narrower at the top or tip and wider atthe base) and pillar shaped (e.g., about the same width at the tip andbase) with circular or hexagonal or other polygonal cross sectionperpendicular to the long axis of the device. Thus, the individualdevices with the completed shells may have various sizes. For example,the sizes may vary, with base widths ranging from 100 nm to several(e.g., 5)μm, such as 100 nm to below 1 micron, and heights ranging froma few 100 nm to several (e.g., 10)μm.

A method of manufacturing a nanowire LED structure in accordance withembodiments of the invention comprises the steps of:

-   -   providing an array of semiconductor nanowires comprising a        second conductivity type (e.g., p-type) region and a first        conductivity type (e.g., n-type) region, the n-type region        extending to the base of the nanowire;    -   depositing a sacrificial layer that completely covers nanowires        in a non-active area and partially covers nanowires in a LED        area, leaving a top portion of the nanowires in the LED area        exposed;    -   depositing a p-electrode on the exposed top portions; and    -   removing the sacrificial layer to obtain an air-bridged        p-electrode.

In prior art methods, arrays of nanowire LEDs are contacted bydepositing a contact layer that covers essentially the whole surface ofthe nanowires and intermediate surfaces between the nanowires usingsputtering or evaporation techniques. Due to the high aspect ratio, andoften small spacing of the nanowires these line-of-sight processesresults in a non-conformal coverage. In particular, there is a risk thatthe contact layer becomes discontinuous and that the contact layer onthe intermediate surfaces (e.g., the horizontal surface exposed betweenvertical nanowires) becomes too thin. In operation, this will result inlosing the effect of some nanowires and a poor current spreading in thedevice, respectively. With an air-bridged p-electrode in accordance withembodiments of the invention, the risk for discontinuities is reduced oreliminated, and the lateral current spreading is improved due to auniform thickness of the p-electrode and optional additional layersdeposited on the p-electrode.

One advantage of an air-bridge p-contact or electrode for top-emittingnanowire LEDs is that a thick contact layer can directly contact the topportion of the nanowire LED. For top emitting nanowire LEDs, atransparent p-contact layer is used. Without the air-bridge, thep-electrode layer at the nanowire top portion must be made much thicker,which increases absorption.

One advantage of the air-bridge p-contact or electrode forbottom-emitting nanowire LEDs is that the reflective p-contact layer isonly arranged on the top portion of the nanowires and not the wholecircumferential nanowire area. A reflective layer extending down on thewhole circumferential area would give significant losses due to totalinternal reflection.

Thus, embodiments of the invention make it possible to obtain anefficient nanowire LED with regard to internal conductivity, lightgeneration and coupling of light out from the nanowire LED.

Embodiments of the invention are defined in the dependent claims. Otherobjects, advantages and novel features of the invention will becomeapparent from the following detailed description of the invention whenconsidered in conjunction with the accompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described withreference to the accompanying drawings, wherein:

FIG. 1 schematically illustrates a side cross sectional view of a basisof a nanowire LED in accordance with embodiments of the invention,

FIG. 2 schematically illustrates a side cross sectional view of ananowire LED structure on a buffer layer in accordance with embodimentsof the invention,

FIGS. 3 a-b schematically illustrate side cross sectional views ofair-bridged p-electrodes in accordance with embodiments of theinvention,

FIGS. 4 a-h schematically illustrate side cross sectional views of afirst implementation of a method in accordance with one embodiment ofthe invention,

FIGS. 4 i-s schematically illustrate side cross sectional views of asecond implementation of a method in accordance with another embodimentof the invention,

FIG. 5 shows two scanning electron microscope images of an air-bridgedp-electrode in accordance with embodiments of the invention,

FIGS. 6 a-i schematically illustrate side cross sectional views of athird implementation of a method in accordance with another embodimentof the invention,

FIG. 7 shows a scanning electron microscope image of a nanowire LEDstructure manufactured according to the third implementation example,

FIG. 8 schematically illustrates a side cross sectional view of an arrayof nanowire LED structures comprising axial pn junctions and anair-bridged p-electrode in accordance with an alternative embodiment ofthe invention,

FIG. 9 illustrates a side cross sectional view of a device according toa further embodiment of the invention,

FIG. 10 shows the side view of the device of the embodiment of FIG. 9mounted on a submount with appropriate contacts,

FIG. 11 illustrates a side cross sectional view of a device according toa still further embodiment of the invention; and

FIG. 12 shows the side view of the device of the embodiment of FIG. 11mounted on a submount with appropriate contacts.

DETAILED DESCRIPTION OF EMBODIMENTS

As used herein, the term “air-bridged electrode” is taken to mean anelectrode structure that extends between adjacent individual devices toleave an empty space between the adjacent devices. The empty space ispreferably surrounded by the adjacent devices on the sides, theair-bridged electrode on the “top” and the support of the devices on the“bottom”, where the terms top and bottom are relative depending on whichway the device is positioned. For example, in one embodiment in whicheach individual device is a radial core-shell nanowire, the air-bridgedelectrode covers the nanowire tips and the space between the nanowires,such that there is an empty space beneath the electrode between thenanowire support layer (e.g., substrate, buffer layer, a reflective ortransparent conductive layer, insulating mask layer, etc.) and theelectrode.

In the art of nanotechnology, nanowires are usually interpreted asnanostructures having a lateral size (e.g., diameter for cylindricalnanowires or width for pyramidal or hexagonal nanowires) of nano-scaleor nanometer dimensions, whereas its longitudinal size is unconstrained.Such nanostructures are commonly also referred to as nanowhiskers,one-dimensional nano-elements, nanorods, nanotubes, etc. Generally,nanowires with a polygonal cross section are considered to have at leasttwo dimensions each of which are not greater than 300 nm. However, thenanowires can have a diameter or width of up to about 1 μm. The onedimensional nature of the nanowires provides unique physical, opticaland electronic properties. These properties can for example be used toform devices utilizing quantum mechanical effects (e.g., using quantumwires) or to form heterostructures of compositionally differentmaterials that usually cannot be combined due to large lattice mismatch.As the term nanowire implies, the one dimensional nature is oftenassociated with an elongated shape. In other words, “one dimensional”refers to a width or diameter less than 1 micron and a length greaterthan 1 micron. Since nanowires may have various cross-sectional shapes,the diameter is intended to refer to the effective diameter. Byeffective diameter, it is meant the average of the major and minor axisof the cross-section of the structure.

FIG. 1 schematically illustrates the basis for a nanowire LED structurein accordance with embodiments of the invention. In principle, onesingle nanowire is enough for forming a nanowire LED, but due to thesmall size, nanowires are preferably arranged in arrays comprisingthousands of nanowires (i.e., nano-devices or devices) side by side toform the LED structure. For illustrative purposes the individualnanowire LED devices will be described herein as being made up fromnanowires 1 having an n-type nanowire core 2 and a p-type shell 3 atleast partly enclosing the nanowire core 2 and an intermediate activelayer 4. However, for the purpose of embodiments of the inventionnanowire LEDs are not limited to this. For example the nanowire core 2,the active layer 4 and the p-type shell 3 may be made up from amultitude of layers or segments. As described above, in alternativeembodiments, only the core 2 may comprise a nanostructure or nanowire byhaving a width or diameter below 1 micron, while the shell 3 may have awidth or diameter above one micron. In order to function as a LED, then-side and p-side of each nanowire 1 has to be contacted.

By growing the nanowires 1 on a growth substrate 5, optionally using agrowth mask 6 (e.g., a nitride layer, such as silicon nitride dielectricmasking layer) to define the position and determine the bottom interfacearea of the nanowires 1, the substrate 5 functions as a carrier for thenanowires 1 that protrude from the substrate 5, at least duringprocessing. The bottom interface area of the nanowires comprises thearea of the core 2 inside each opening in the masking layer 6. Thesubstrate 5 may comprise different materials such as III-V or II-VIsemiconductors, Si, Ge, Al₂O₃, SiC, Quartz, glass, etc., as discussed inSwedish patent application SE 1050700-2 (assigned to GLO AB), which isincorporated by reference herein in its entirety. In one embodiment, thenanowires 1 are grown directly on the growth substrate 5.

Preferably, the substrate 5 is also adapted to function as a currenttransport layer connecting to the n-side of each nanowire 1. This can beaccomplished by having a substrate 5 that comprises a buffer layer 7arranged on the surface of the substrate 5 facing the nanowires 1, asshown in FIG. 2, by way of example a III-nitride layer, such as a GaNand/or AlGaN buffer layer 7 on a Si substrate 5. The buffer layer 7 isusually matched to the desired nanowire material, and thus functions asa growth template in the fabrication process. For an n-type core 2, thebuffer layer 7 is preferably also doped n-type. The buffer layer 7 maycomprise a single layer (e.g., GaN), several sublayers (e.g., GaN andAlGaN) or a graded layer which is graded from high Al content AlGaN to alower Al content AlGaN or GaN. The nanowires can comprise anysemiconductor material, but for nanowire LEDs III-V semiconductors suchas a III-nitride semiconductor (e.g., GaN, AlInGaN, AlGaN and InGaN,etc.) or other semiconductors (e.g., InP, GaAs) are usually preferred.It should be noted that the nanowire 1 may comprise several differentmaterials (e.g., GaN core, InGaN active layer and InGaN shell having adifferent In to Ga ratio than the active layer). In general thesubstrate 5 and/or the buffer layer 7 are referred to herein as asupport or a support layer for the nanowires. As will be described inmore detail with regard to FIGS. 9-12, a conductive layer (e.g., amirror or transparent contact) may be used as a support instead of or inaddition to the substrate 5 and/or the buffer layer 7. Thus, the term“support layer” or “support” may include any one or more of theseelements.

Thus, the buffer layer 7 provides means for contacting the n-side of thenanowires 1. In prior art nanowire LEDs, the contacting of the p-side ofeach nanowire 1 is typically accomplished by depositing a p-electrodecomprising a conductive layer that encloses the p-type shell 3 of eachnanowire 1 and extends to an insulating layer on the substrate or bufferlayer. The conductive layer extends on this insulating layer to adjacentnanowires. However, since the nanowires of a nanowire LED are closelyspaced and being of high aspect ratio in order to obtain a highluminescence, the p-electrode deposition is a challenging operation.Typically line-of-sight processes, such as sputtering or evaporation areused for electrode deposition. Due to the line-of-side deposition, apreferential growth on the tips of the nanowires and a shadowing effectare observed that result in a tapering of the p-electrode with decreasedthickness towards the base of the nanowires 1. Hence, in order to obtainefficient lateral current spreading, the thickness of the p-electrodewill become unnecessarily thick on the tips of the nanowires while beinginsufficiently thick in between the nanowires. The shadowing effect mayalso be so severe that there are discontinuities in the p-electrode.

A p-electrode 8 in accordance with embodiments of the invention is atleast partly air-bridged between adjacent nanowires 1. FIG. 3 aschematically illustrates a p-electrode 8 covering a group of nanowires1. As noted above, if the shell 3 of the nanowires 1 is n-type, thenelectrode 8 would be an n-electrode. However, electrode 8 is referred toherein as p-electrode for ease of description. The p-electrode 8 isfree-hanging between adjacent nanowires 1 and is only supported by thenanowires 1. The p-electrode 8 encloses a top portion of each nanowire 1and thereby contacts the p-side of the nanowire LED structure. Thep-electrode may extend down along the sides of peripheral nanowires,e.g., in order to provide a connection to a pad arranged on thesubstrate 5 (as will be described in more detail below and as shown onthe right and left edges of FIG. 3 a).

Different additional layers may be deposited on the p-electrode. Forexample layers that improve electrical conductivity or coupling of lightout from/into the nanowire may be deposited on the nanowire.

The nanowire LED structure of the embodiments of the present inventionis either adapted for top emitting, i.e., light emission through thep-electrode, or bottom emitting, i.e., light emission through thesupport layer (i.e., through the conductive layer and/or buffer layerand/or substrate). The requirements on the p-electrode are different forthese two cases. As used herein, the term light emission includes bothvisible light (e.g., blue or violet light) as well as UV or IRradiation.

For a top emitting device, the p-electrode needs to be transparent(i.e., it should transmit the majority of light emitted by the LED).Indium Tin Oxide (ITO) is a suitable material for the p-electrode, inparticular for the top emitting nanowire LED. The ITO preferably has athickness of 150-900 nm, more preferably 250-650 nm, most preferablyabout 500 nm. ITO has been extensively used also for planar componentswhere LED devices are formed by layer-by-layer techniques. In suchcomponents the thickness of the ITO layer is preferably about 150 nmsince this is enough to obtain an acceptable current spreading. Onedrawback with increased thickness is that the ITO has comparatively highabsorption of light. Therefore the ITO thickness is kept as low aspossible. Another reason for keeping the ITO thickness low is that theITO cannot readily be wet etched if too thick, i.e. more than 150 nm.Surprisingly the optimal ITO thickness for the p-electrode in accordancewith the embodiments of then present invention is high. This can beexplained by the fact that the efficiency of the nanowire LED isdetermined by a trade-off between good light coupling, i.e.comparatively thick ITO, and low absorption, i.e. comparatively thinITO. The ITO can also be combined with layers of other materials toobtain specific properties. For example, similar properties as whenhaving a 500 nm ITO can be obtained by having a 150 nm ITO covered witha silicon oxide layer. A thick p-electrode, preferably uniformly thick,will also contribute to efficient heat dissipation.

Other suitable materials for a p-electrode on a top emitting device areZnO, doped ZnO and other transparent conducting oxides (TCOs). Importantparameters for this material are good transparency, electricalconductivity and the ability to make low resistive contact to the shell.High thermal conductivity is also desirable, together with a matchingrefractive index (depending on configuration).

In one embodiment of a top emitting nanostructured LED the substrate isprovided with a reflecting means (e.g., mirror) that preferably extendsin a plane under the nanowire LEDs.

For a bottom emitting LED, the p-electrode is preferably reflective. Asshown in the following examples, the p-electrode may comprise one ormore additional layers deposited on the p-electrode for improving thereflective and/or conductive properties.

FIG. 3 b schematically illustrates one embodiment of a nanowire LEDstructure in accordance with embodiments of the invention. In principleit is the same structure as shown in FIG. 3 a, but the p-electrodecomprises a comparatively thin conductive layer 8′ enclosing the p-typeshell 3 and a comparatively thick air-bridged conductive layer 8″arranged on the thin conductive layer 8′. The thin conductive layer 8′extends down towards the base of the nanowire 1, longer than the thickconductive layer 8″. The thin conductive layer 8′ can for example bedeposited using atomic layer deposition or grown as an epitaxial layeron the p-type shell. Layer 8′ may be discontinuous between adjacentnanowires and may cover only the nanowires but not the masking layer 6or buffer layer 7 between the nanowires. With this arrangement the thinconductive layer 8′ can be used to obtain an optimal interface to thep-type shell 3 and the thick conductive layer 8″ can be optimised forcurrent spreading and/or light coupling and/or reflection. Thus, onlylayer 8″ may be used to form the air-bridge.

In an alternative embodiment, in addition to the mask layer 6, the spacebetween the nanowires can also be filled fully or partially with adielectric (i.e., insulating) material, such as silicon oxide. Forpartially filled space, the air gap size below the air-bridge isreduced. For fully filled space, there is no longer an air-bridge. Thus,for the embodiments described below with regard to the contact schemesfor the nanowires, it should be understood than the nanowires may becontacted either in an air-bridged or non-air-bridged configurations.

In the following first implementation of a method for forming a topemitting nanowire LED structure is described with reference to FIGS. 4a-h. In this embodiment, the same conductive layer is patterned to formboth the p and n electrode layers. In this implementation, pads forconnecting to the n-side and p-side of the device are formed in padareas adjacent to the nanowires forming the nanowire LED. However, theinvention is not limited to this configuration.

FIG. 4 a shows an array of nanowires 1 grown from a buffer layer 7through a growth masking layer 6. The nanowires preferably comprise ann-type nanowire core 2 enclosed in a p-type shell layer 3 with anintermediate active layer 4 for light generation, as shown in FIG. 1.The growth mask 6 may be patterned by photolithography to defineopenings for the nanowire growth, as described for example in U.S. Pat.No. 7,829,443. In this implementation, the nanowires are grouped in ann-pad area, a non-active area, a LED area (i.e., the area which emitslight) and a p-pad area. However, embodiments of the invention are notlimited to this. For example the p-pad area may be arranged on top ofthe nanowires forming the light emitting part of the nanowire LEDstructure, whereby the p-pad area and the LED area coincide, asdescribed in PCT International Application Publication Number WO2010/014032 A1 to Konsek, et al., published Feb. 4, 2010 andincorporated herein by reference in its entirety.

Referring to FIG. 4 b, in the next step a protection layer 9 isdeposited, at least in the LED-area where the nanowires will form LEDs,to protect nanowires from the subsequent processing. Otherwise residualsfrom photoresist and reactive ions from sputtering and reactive ionetching (RIE) may cause defects and/or contamination. ZnO deposited withatomic layer deposition (ALD) can be used as a protection layer. Oneadvantage with ALD as a deposition technique is its perfect stepcoverage. Other materials such as other metal or silicon oxides, e.g.,Al₂O₃ or SiO₂, deposited with ALD or other deposition techniques canalso be used. This layer may fill an additional role as insulator onareas where it will be left.

Protection layer 9 deposition is followed by opening up, throughlithography and etching, to the buffer layer 7 through the protectionlayer and the growth mask in the n-pad area 11. In other words, as shownin FIG. 4 b, a photoresist or another masking layer (shown as dashedlines 12) is formed over the entire device and then removed in the n-padarea 11 by photolithography. The exposed protection layer 9 and theexposed masking layer 6 between the nanowires 1 are etched by anysuitable etching method which can stop on the buffer layer (e.g., anywet or dry etching method which can etch a metal oxide or silicon oxideselectively with respect to a III-nitride semiconductor buffer layer).The purpose is to access the buffer layer 7 for arranging an electrodethereon, in order to provide an electrical connection through the bufferlayer 7 to the n-side of the nanowires 1 (i.e., an electrical connectionthrough n-type layer 7 to n-type nanowire cores 2).

Referring to FIG. 4 c, the next step is to form a sacrificial layer 10,such as a photoresist or another suitable sacrificial material layer,with two different thicknesses extending over the non-active area andthe p-pad area. The photoresist layer should completely cover thenanowires in the non-active area 13, whereas it should partly cover thenanowires 1 in the LED area 14, leaving a top portion of each nanowire 1in the LED area 14 exposed. If the same contact materials are to be usedin the n- and p-electrodes, areas which should be accessed as contactpads, i.e., the n-pad area 11 and the p-pad area 15, the n-pad area is11 preferably not covered by photoresist. This is clearly seen in theleft part of FIG. 4 c. As appreciated by one skilled in the art, thisphotoresist layer can be formed e.g. by depositing photoresist, andusing two masks and two exposures and then development, or largeramplitude exposure in area 14 than in area 13 for a positive photoresist(or vice versa for negative photoresist). Also, the photoresist cancomprise multiple layers (e.g., forming, exposing and developing a firstresist in areas 13 and 14 and then forming, exposing and developing asecond resist only in area 13 over the first resist). If desired, thephotoresist 10 may comprise a portion of the photoresist layer 12 usedin FIG. 4 b to pattern layers 9 and 6. In this case, photoresist layer12 is exposed a second time using the methods described above in areas14 and 15 but not in area 13 (or vice versa for a negative photoresist),and then developed (i.e., removed) fully in area 15 and partially inarea 14.

Referring to FIG. 4 d, the next step is to remove the protection layer 9on at least the exposed top portions of the nanowires in the LED area 14that are exposed outside of photoresist pattern 10. This may be done byselective etching which selectively etches the oxide protective layer 9but not the masking layer 6 (e.g., silicon nitride) or the semiconductorbuffer layer 7 or semiconductor nanowires 1. If desired, layer 9 may beleft in areas where it does not interfere in a contact between thesemiconductor material and a respective electrode to provide additionalelectrical insulation on top of the masking layer 6. For example, analuminium oxide layer may be used as such a permanent protective layer 9in combination with silicon nitride masking layer 6.

Thereafter the p-electrode layer 16 is deposited. Since the p-electrodebecomes elevated and does not have to extend down deeply into the narrowspace between the nanowires 1, line-of-sight processes such assputtering or evaporation can be used. Of course the n-electrode layeris formed at the same time since the n-pad area 11 is exposed. It shouldbe noted that p-electrode 16 does not contact the n-type buffer layer 7in the p-pad area 15 because the buffer layer 7 is covered by themasking layer 6 in the p-pad area. Thus, a short circuit between thep-electrode and the n-buffer layer/n-nanowire cores is avoided. However,if the left side portion of layer 16 is used to form the n-electrode,then it this portion of layer 16 contacts the exposed buffer layer 7between the nanowires in the n-pad area 11. It should be noted thatlayer 16 does not contact the nanowires 1 in the non-active area 13which is covered by the photoresist 13.

Referring to FIG. 4 e, next step is to do another lithography stepleaving another photoresist pattern 17 in the p-pad area 15, the LEDarea 14 and the n-pad area 11. This may be done by forming anotherphotoresist layer over the device shown in FIG. 4 d (including over themetal electrode 16 covered resist pattern 10 in non-active area 13) andthen exposing and developing the photoresist to leave the photoresistpattern 17 on both sides of the metal electrode 16 covered resistpattern 10.

Referring to FIG. 4 f, the next step is to remove the electrode material16 on the areas where electrode material is not covered by resistpattern 17 from the previous step, i.e., in the non-active area 13,which can be done by selective dry or wet etching which does not removethe photoresist patterns 10 and 17. This causes the electrode layer 16to become discontinuous such that it is removed in the non-active area13 between the n-pad area 11 and the active and p-pad areas 14, 15.

Referring to FIG. 4 g, next step is removal of all remaining photoresist10, 17, which can be done by dissolving and/or plasma etching. Thisleaves the p-electrode layer 16 free-hanging between the nanowires 1 inthe LED area 14. This forms the air-bridge with empty space 18 betweenthe electrode 16, nanowires 1 and the masking layer 6.

Referring to FIG. 4 h, finally residues of the protective layer 9 on thenon-active area 13 that still may be present is removed. Thus, layer 16forms the p-electrode 16 a which contacts the tips of the nanowire 1p-shells 3 and contacts the masking layer 6 in the p-pad area, as wellas the n-electrode 16 b which contacts the n-buffer layer 7 in the n-padarea 11. FIG. 5 shows two scanning electron microscope images of anair-bridged p-electrode in accordance with this embodiment of theinvention. The intersection between the non-active area and LED areawith the air-bridged p-electrode is visible on the left.

Since layer 16 was removed in non-active area 13, the same layer 16 maybe used to form both p- and n-electrodes. Thus, in the above processsequence illustrated by FIGS. 4 a to 4 h, the p-electrode andn-electrode are deposited in the same step. The n-electrode layer 16 bcomprises an n-pad area 11 on a first part of the buffer layer 7. Thep-electrode layer 16 a comprises a p-pad area 15 on the nanowires in aLED active area 14 or on a dielectric masking layer 6 on the bufferlayer 7 adjacent to the nanowires in the LED active area. The n-pad areaand the p-pad area are separated by a non active area 13 comprisingdummy nanowires 1 which do not contact the p-electrode (i.e., thesenanowires do not emit light).

However, in an alternative second embodiment, the p-electrode isprovided in a first step and the n-electrode is formed from a differentmaterial at a later stage. Such a process is discloses in FIGS. 4 i to 4s and will be briefly described below. The description of the sameelements and steps from FIGS. 4 a-4 h will not be repeated below forbrevity.

The first two steps in the second embodiment method are identical to thefirst embodiment method, i.e. FIGS. 4 a and b represent the same stepsas FIGS. 4 i and j. However, the protective layer 9 and masking layer 6are not removed in the n-pad area 11 FIG. 4 j as in FIG. 4 b.

In the next step, a sacrificial (e.g., resist) layer 10 a is depositedin two different thicknesses such that no nanowires are left uncoveredin the n-pad area 11 as in the first embodiment. Thus, in the left handside of FIG. 4 k it can be seen that the nanowires in area 11 areentirely covered just as the central nanowires in the non-active region13, as opposed to in FIG. 4 c where the leftmost nanowires in n-pad area11 are completely uncovered. The nanowires in the LED area 14 arepartially exposed on the top in the photoresist 10 a. The p-pad area 15is completely exposed in photoresist pattern 10 a.

FIG. 4 l shows that the protective layer 9 is at least partially removedfrom the exposed nanowire tips in LED area 13 in order to provide forcontact between the p-shell 3 of the nanowires in area 13 and thep-electrode.

The p-electrode layer 16 is then deposited as shown in FIG. 4 m. Layer16 covers the entire structure. The inactive 13 and n-contact 11 areasare now covered by the photoresist 10 a and layer 16 is formed on top ofthe photoresist 10 a. Layer 16 contacts the exposed p-shells 3 of thenanowires in LED area 14 and the masking layer 6 in the p-pad area 15.

As shown in FIG. 4 n, a second photoresist pattern 17 a is now providedover the p-electrode layer 16 in the LED area 14 and the p-pad area 15.Photoresist pattern 17 a is removed in areas 13 and 11. Thus, layer 16is exposed in areas 11 and 13.

The exposed p-electrode layer 16 is then removed from areas 11 and 13 byselective etching, as shown in FIG. 4 o.

As shown in FIG. 4 p, all photoresist 10 a, 17 a is removed such thatthe p-electrode layer 16 forms an air-bridge with underlying emptyspaces 18 between the nanowires in LED area 14, and forms a p-contactpad in area 15.

Next, a new photoresist pattern 19 is applied to cover areas 13, 14 and15 but not the n-pad area 11, as can be seen in FIG. 4 q. The protectivelayer 9 and masking layer 6 are removed from exposed area 11.

N-electrode layer 20 is then deposited over the entire structure, asshown in FIG. 4 r. Layer 20 may comprise Ti and Al sublayers or anyother suitable metal. Layer 20 contacts the exposed buffer layer 7 and“dummy” shorted nanowires in area 11. Layer 20 rests on photoresist 19in areas 13, 14 and 15.

FIG. 4 s shows a lift-off step in which the photoresist pattern 19 isremoved to lift off layer 20 in areas 13, 14 and 15, such that theremaining layer 20 in area 11 forms the n-electrode. There is noelectrode layers 16, 20 in the non-active area 13. This preventsshorting of layer 16 and 20. Dummy nanowires are located in thenon-active area 13.

FIGS. 4 h and 4 s show in process devices prior to formation of contacts(e.g., lead wires or bump electrodes) to the p-electrode 16 a, 16 andn-electrode 16 b, 20, respectively. However, it should be understoodthat the contacts described with respect to FIG. 6, 10 or 12 are made tothe p-electrode in p-pad area 15 and the n-pad area 11, respectively.Furthermore, as noted above, the p-pad area 15 may be on top ofnanowires (e.g., areas 14 and 15 are combined) rather than betweennanowires as shown in FIGS. 4 h and 4 s.

The following third implementation of a method for forming a bottomemitting nanowire LED structure is described with reference to FIGS. 6a-h. In this implementation pads for connecting to the n-side and p-sideare again formed in n-pad areas and p-pad areas, respectively, adjacentto the nanowires forming the nanowire LED. However, the invention is notlimited to this. The same elements that were described above will not bedescribed again below for brevity.

FIG. 6 a shows the structure which is similar to FIG. 4 a. As in thepreviously described with respect to FIG. 4 a, a photoresist layer orpattern 10 b with two thicknesses completely covers nanowires in thenon-active area 13 and partially encloses the nanowires in the LED area14, leaving the top nanowire portions exposed. The n-pad area 11 and thep-pad area 15 are open and not covered by the photoresist pattern 10 b.

Referring to FIG. 6 b, in a next step, the protective layer 9 on theexposed top portion of the nanowires in the LED area 14 is selectivelyremoved. Then, a p-electrode layer 16 c, a current spreading layer 16 dand one or more reflector layers 16 e are deposited over the entiredevice by for instance sputtering or evaporation. One or more of theselayers may be omitted (e.g., the reflector layer 16 d may be omitted ifa separate mirror will be used), as long as at least one conductivelayer is formed.

Referring to FIG. 6 c, in a next step the photoresist 10 b is removed tolift off layers 16 c, 16 d and 16 e, and is optionally followed by aheat treatment to tune the properties of the layers. This leaves layers16 c-16 e in areas 11, 14 and 15. An air bridge is formed in area 14with empty spaces 18 described above. This separates layer 16 c-e intop-electrode 22 and n-electrodes 23, as shown in FIG. 6 d.

Referring to FIG. 6 d, in next step, the residues of the protectivelayer 9, on the non-active area 13 is removed if desired.

Referring to FIG. 6 e, in a next step solder ball bumps (SBB) (e.g.,p-bump 21 a and n-bump 21 b) are attached to the p-pad 15 and n-pad 11areas, respectively. In the p-pad area 15, the p-electrode 22 isisolated from the n-buffer layer 7 by the masking layer 6. Thep-electrode 22 provides electrical contact between the p-bump 21 a andthe p-shells 3 in area 14. The n-electrode 23 provides contact betweenn-bump 21 b and the n-buffer layer 7 and n-cores 2. Thus, the bufferlayer is accessed by the n-electrode/n-bump and the shells are accessedby the p-electrode/p-bump to provide an external electrical connectionto the LEDs.

Referring to FIG. 6 f, in a next step the chip, i.e., the LED structure,is flipped over and dipped in a conductive adhesive 23 which remains onthe bumps 21 a, 21 b. In addition to providing electrical conductivity,the conductive adhesive may improve the heat dissipation properties.

Referring to FIG. 6 g, in a next step the chip is mounted on a carrier24 pre-processed with p- and n-electrodes 25 and 26. Although describedin terms of a SBB arrangement it is appreciated by a person skilled inthe art that there are other contact alternatives, such as lead wire orlead frame connections.

Referring to FIG. 6 h, in a next step, the space between the chip andthe carrier is underfilled, for example by an epoxy material 27. Theunderfill provides structural rigidity and may also contribute toimproved heat dissipation.

Referring to FIG. 6 i, in a next step the Si substrate 5 is removedcompletely or partially by for example wet or dry etching to form anopening 28 exposing the buffer layer 7. If desired, the buffer layer 7may also be removed through opening 28 to expose the nanowire 1 bases.

FIG. 7 shows a nanowire structure obtained by this implementation of themethod with a solder bump arranged on the nanowires. The p- andn-electrodes are accessed via the carrier wafer using a p-contact 29 andan n-contact 30. This forms a bottom emitting LED device which emitslight from LED areas 14 through the buffer layer 7.

As mentioned above, nanowires may comprise heterostructures ofcompositionally different materials, conductivity type and/or dopingsuch as the above exemplified radial heterostructures forming the pn orpin junction. In addition, axial heterostructures within the nanowirecore may also be formed. These axial heterostructures can form pn- orp-i-n-junctions that can be used for light generation in a nanowire LED.FIG. 8 schematically illustrates a plurality of nanowires with axialpn-junctions (e.g., the p-portion 3 located above the n-portion 2 in theaxial direction) contacted on the p-side 3 with an air-bridge electrode8 arrangement.

Although the present invention is described in terms of contacting ofnanowire LEDs, it should be appreciated that other nanowire basedsemiconductor devices, such as field-effect transistors, diodes and, inparticular, devices involving light absorption or light generation, suchas, photodetectors, solar cells, lasers, etc., can be contacted in thesame way, and in particular the air-bridge arrangement can beimplemented on any nanowire structures.

All references to top, bottom, base, lateral, etc are introduced for theeasy of understanding only, and should not be considered as limiting tospecific orientation. Furthermore, the dimensions of the structures inthe drawings are not necessarily to scale.

Further embodiments of the invention provide processes for packaging topemitting LEDs as disclosed above, and such processes will be describedbelow with reference to FIGS. 9-12.

Reference is first made to FIG. 9, showing a top emitting nanowire LEDstructure 90 having a mirror 91 provided on the backside. Thus, asalready mentioned, for top emitting LEDs the p-contact 92(6) istransparent, preferably made of TCO, conductive polymer or thin metaland in order to direct the emitted light through the top, a mirror ispreferably provided below.

In order to attach a mirror, different methods can be used. In oneembodiment, after the p-contact 92 has been provided, an n-contact93(20) is provided on selected n-contact areas 11 on the LED array onthe tips of the nanowires and between the nanowires in contact with thebuffer layer 96 in the selected area 11 to provide a base for later wirebonding. The n-contact, suitably made of Ti/Al or other conductors(e.g., transparent alternatives, such as TCO), is deposited so as tocover the entire nanowire 94(1) and down through the masking layer 95(6)so as to contact the buffer layer 96(7), whereby after suitable wirebonding can be applied to the LED array. The deposition of the n-contactcan be performed by any deposition technique, exemplified by sputtering,thermal or e-beam evaporation and plating. In order to make contact forthe Ti/Al, openings are made in the masking layer 95 by etching e.g.,wet etching or by dry etching (RIE).

In order to enable handling during the further processing of the array,a temporary carrier C is bonded to the p-contact side. The carrier isschematically shown in ghost lines. This carrier is suitably a siliconwafer, a ceramic substrate, or a glass or metal plate. There are severalmethods for achieving the bonding of the carrier, such as a productobtainable from Brewer Science called “Temporary wafer bonding” orcontact bonding. Other alternatives are to use photoresist, BCB or someother polymer temporary adhesive material. The adhesive is coated on thecomponents to be bonded and pressure (and optionally heating) isapplied. When the carrier is attached, the original growth substrate 5on which the nanowires were grown is subjected to an etching procedureto provide a recess 97, such as an opening, depression or full removalof the growth substrate down to the GaN/AlGaN buffer layer, i.e. on theback side of the array. The width of the recess is indicated by thebracket. Etching is suitably by dry etching, e.g., the so called BoschProcess, well known to the skilled artisan. It is also possible to usewet chemical methods or a combination of grinding and etching.

A further step of removing (partly or entirely) the GaN/AlGaN bufferlayer and replacing it with a conductive layer (e.g., 91) can optionallybe made at this point. In top emitting applications this layer canpreferably comprise a mirror, but should be transparent for bottomemitting applications.

If, the growth substrate is only partly removed, in order to protect theareas of the substrate which should not form the depression, suitablemasks (e.g., photoresist) are applied, depending on the etch methodused.

The mirror 91 is provided in the recess 97 for a top emitting LED. Ifdesired, plural mirrors may be formed in plural recesses in thesubstrate. The provision of the mirror can be performed in severaldifferent ways. A preferred method is by sputtering Ag into the recess97 to a thickness of about 1 μm. Thicker layers improve thermalconductivity but usually do not improve reflectivity appreciably.Alternatively methods such as thermal or e-beam evaporation or platingcan be used. This mirror can be passive in the sense that it is notelectrically active in the array. The mirror simply reflects light outfrom the array to the top of the nanowires. It can also be active, butthen additional steps, such as thinning of resistive layers should bemade if the buffer ends with a material such as AlGaN.

After the mirror has been provided, the recess 97 is filled up with afiller material 98, e.g. epoxy or other suitable heat conductivematerial, preferably of high mechanical strength, so as to providestructural rigidity. Other appropriate materials are exemplified by, butnot limited to TiN, graphene, and other polycrystalline or amorphouscarbon films. Such materials may be most suitable in cases where thegrowth substrate is fully removed, as deposition times may be aconstraint when used in deep recess structures. Then, the entirestructure is debonded from the temporary carrier C to arrive at thestructure shown in FIG. 9.

The structure thus obtained is mounted to a suitable mount structure orcarrier 100, as shown in FIG. 10. The mount structure surface maycomprise layers to enhance adhesion or improve thermal properties of thedevice such as metals, TiN, graphene, and other polycrystalline oramorphous carbon films. Contacts pads, preferably gold or otherconductive materials 101 are deposited on the n-contact areas to providea bond pad for wire bonding. Wires 102, such as gold or other wires arethen attached between the mount 100 and pad 101. Solder bumps or bondsmay be used to secure the wire 102 in place. At least one p-contact pad103 is also provided on the p-electrode and wire 104 bonding is alsomade to this contact from the mount 100. Finally, there is provided aprotective “bulb” or package 105 over the exposed LEDs, suitably of asilicone material.

Now, with reference to FIG. 11, a variant for the provision of a passivemirror is shown. P- and n-contacts 92, 93, respectively, are provided asdescribed above with respect to FIG. 9, and a temporary carrier C isalso attached in the same manner. However, after the temporary carrierhas been provided, the original substrate 5 is removed, suitably byetching, although methods such as polishing or grinding could be usedtoo.

After the removal of the substrate so as to expose the buffer layer(e.g., AlGaN) 7, a mirror 111 is provided on the buffer layer 7.Suitably an Ag mirror is provided by sputtering. Then, the assembly isglue bonded to a new substrate 112, preferably using any of silicone,epoxy, BCB or other types of polymer 113 as adhesive. Finally, thecarrier C is removed.

The same process steps as in the previous embodiment are used to makecontacts, wire bonding and the protective “bulb” 105, as shown in FIG.12.

In a variation of the process just described in connection with FIGS. 11and 12, the same process is used except that instead ofapplying/attaching a mirror to the buffer layer, instead the mirror 111is applied to a new substrate 112. Then the assembly of new substrateand mirror is attached to the buffer layer of the LED array structure byglue bonding. This of course requires that the adhesive is practicallyfully transparent in order that undue losses will not occur. Theadhesive 113 in this case is located between the mirror 111 and thebuffer layer 7. In these embodiments, the mirror 91, 111 is used as thesupport for the nanowires 1 in addition to and/or instead of thesubstrate 5 and/or buffer layer 7.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiments,it is to be understood that the invention is not to be limited to thedisclosed embodiments, on the contrary, it is intended to cover variousmodifications and equivalent arrangements within the scope of theappended claims.

The invention claimed is:
 1. A light emitting diode (LED) structure,comprising: a plurality of devices arranged side by side on a supportlayer, wherein each device comprises a first conductivity typesemiconductor nanowire core and an enclosing second conductivity typesemiconductor shell for forming a pn or pin junction that in operationprovides an active region for light generation, and a first electrodelayer that extends over the plurality of devices and is in electricalcontact with at least a top portion of the devices to connect to theshell, wherein the first electrode layer is at least partly air-bridgedbetween the devices, wherein the support layer comprises a n-typesemiconductor buffer layer on a substrate, which buffer layer serves asn-contact, and an n-electrode layer which contact the buffer layer. 2.The light emitting diode (LED) structure of claim 1, wherein the firstconductivity type comprises n-type, the second conductivity typecomprises p-type and the first electrode layer comprises a p-electrodelayer.
 3. The light emitting diode (LED) structure of claim 2, furthercomprising a second n-electrode layer which electrically connects to then-type nanowire cores.
 4. The light emitting diode (LED) structure ofclaim 1, wherein the support layer is reflective.
 5. The light emittingdiode (LED) structure of claim 1, wherein the support layer istransparent.
 6. The light emitting diode (LED) array of claim 2, whereinthe thickness of the air-bridged p-electrode layer is 150 nm-900 nm. 7.The light emitting diode (LED) structure of claim 1, wherein thep-electrode is at least partly reflective.
 8. The light emitting diode(LED) structure of claim 1, wherein the p-electrode is transparent. 9.The light emitting diode (LED) structure of claim 8, wherein at leastone other transparent layer is arranged on the p-electrode.
 10. Thelight emitting diode (LED) structure of claim 1, wherein the LED isflip-chip bonded onto contact electrodes on a carrier.
 11. The lightemitting diode (LED) structure of claim 1, further comprising a mirrorprovided below the nanowire cores.
 12. The light emitting diode (LED)structure of claim 11, wherein the mirror is provided in a recess formedin the substrate, said recess extending to the buffer layer.
 13. Thelight emitting diode (LED) structure of claim 12, further comprising aplurality of mirrors in a plurality of recesses.
 14. The light emittingdiode (LED) structure of claim 12, wherein the mirror comprises areflective material that partially fills said recess, and wherein aremaining portion of the recess is filled with a filler materialproviding structural rigidity and high thermal conductivity.
 15. Thelight emitting diode (LED) structure of claim 11, wherein the mirror isglue bonded to one of the buffer layer and the substrate, and physicallybonded to other of the buffer layer and the substrate.
 16. The lightemitting diode (LED) structure of claim 4, wherein the support layercomprises a mirror and wherein the mirror is provided in place of aremoved n-type semiconductor buffer layer and fully removed substrate.17. The light emitting diode (LED) structure of claim 2, wherein eachdevice comprises a nanostructure containing the core, the shell and anactive layer between the core and the shell.
 18. The light emittingdiode (LED) structure of claim 1, wherein the nanostructure comprises acore-shell nanowire.
 19. The light emitting diode (LED) structure ofclaim 1, wherein the support layer comprises a semiconductor substrate.20. The light emitting diode (LED) structure of claim 3, wherein: then-electrode layer comprises an n-pad area on a first part of the bufferlayer; the p-electrode layer comprises a p-pad area on the nanowires ina LED active area or on a dielectric masking layer on the buffer layeradjacent to the nanowires in the LED active area; and the n-pad area andthe p-pad area are separated by a non active area comprising dummynanowires which do not contact the p-electrode.
 21. A top emitting lightemitting diode (LED) structure, comprising: a plurality of deviceslocated on a first surface of a support layer, the devices comprising asemiconductor nanowire core of a first conductivity type and asemiconductor shell of a second conductivity type; a first transparentelectrode which is electrically connected to the shells of the devices,wherein the first electrode layer is at least partly air-bridged betweenthe devices; a second electrode located in electrical contact with thefirst surface of the support layer and electrically connected to thecores of the devices through the support layer; a carrier attached tothe device; a first contact electrically connecting the carrier to afirst pad area of the first electrode; a second contact electricallyconnecting the carrier to a second pad area of the second electrode; anda reflective layer located below the cores of the devices, wherein thecarrier substrate is located below the reflective layer.
 22. The lightemitting diode (LED) structure of claim 21, wherein: the nanowires havea base end and a tip end; the base end is attached to the support layercomprising a growth substrate or a buffer layer; and the first electrodeextends from the tip end of the nanowires down between the nanowires.23. The light emitting diode (LED) structure of claim 22, wherein thedevice comprises an air gap in spaces between the nanowires between thefirst electrode and the support layer.
 24. The light emitting diode(LED) structure of claim 21, wherein the first conductivity type isn-type, the second conductivity type is p-type, and the semiconductorcore and the semiconductor shell comprise a III-nitride semiconductormaterial nanowire.
 25. The light emitting diode (LED) structure of claim21, further comprising a dielectric masking layer with openings locatedon the first surface of the support layer such that the coreselectrically contact the first surface of the support layer through theopenings and the shells are electrically isolated from the first surfaceof the support layer by the masking layer.
 26. The light emitting diode(LED) structure of claim 25, wherein: the first electrode pad area islocated between the nanowires on the masking layer; and the secondelectrode pad area is located on the first surface of the support layerbetween the nanowires in an opening in the masking layer.